Starch and Cellulose Starch

Starch is the major reserve carbohydrate in plants. Potato, maize, cassava and wheat provide the main sources of energy in the human diet, but also serve for many industrial processes like adhesives, cosmetics, detergents, paper, textiles and pharmaceuticals (Davis et al. 2003). Starch is also used for the production of biodegradable plastics as an alternative to petroleum-based products. However, native starches from various plant species have limited physiochemical properties, and thus are directly suitable for only a few specific end uses. For many industrial uses, enzymatic and chemical treatments, it is necessary to improve the usability of starch. The modification of starch is possible by using biotechnology to alter starch composition or to modify starch synthesis (Chap. 11). Out of many examples for starch modification, we describe one example for altered starch composition.

Starch is composed of amylopectin and amylose, which have different characteristics for industrial purposes. Amylopectin is used as a thickener, while amylose is undesirable for many products and can interfere with certain processes. Therefore a transgenic starch potato was developed which produces exclusively the amylopectin component of starch (Kull et al. 1995). In order to do so, the gene encoding the granule-bound starch synthase (GBSS) for the biosynthesis of amylose was inactivated by post-transcriptional gene silencing (PTGS). Two subgenomic fragments of the gene were expressed in antisense orientation under control of the CaMV 35S promoter. The resulting transgenic potato plants were effective in inhibiting amylose biosynthesis in tubers, thereby leading to an increase in the branched starch component amylopectin (>98%). The phenotype was stable during vegetative propagation. For commercial use the potato variety was named "Amflora" and has been analysed in field trials for several years to test yields and resistance to pests and disease. Furthermore the allergic and toxic potential of Amflora tubers was analysed, as well as potential other impacts on human health and the environment. No increased risk to humans, animals and the environment were shown in comparison to conventional potatoes (EFSA 2005).

13.2.1.2 Cellulose

All cell walls of higher plants contain: (i) cellulose, ahomopolymer of p-1,4-linked glucose units, which is a flexible structural substance in the form of fibrils, (ii) hemicellulose, a heterogeneous polysaccharide, which represents a matrix in which the cellulose fibrils are embedded, and (iii) lignin, a phenol polymer, which forms a bond between cellulose and hemicellulose. To date, cellulose is mainly isolated from trees. In order to obtain pure cellulose fibres (e.g. for the paper industry) lignins have to be eliminated. The chemical process is very expensive and pollutive under high energy consumption (Franke et al. 2000), hence reducing the lignin contents in trees might ease the isolation of cellulose.

Since the beginning of 1990 several strategies were investigated for plants to reduce lignin contents using gene technology. Mostly poplars were investigated, because there are fast-growing trees, relatively easy to modify and play an important role in paper manufacture. Up to now several effects have been achieved, due to the modification of biosynthetic pathway steps (Fig. 13.1; Pickardt and de Kathen 2004).

  1. Decrease of complete lignin content by about 45%. Furthermore cellulose content was increased up to 14% (Hu et al. 1999).
  2. Increase of the lignin compounds sinapyl to coniferyl units through overexpression of coniferaldehyde-5-hydroxylase. The total lignin content is equal.
  3. Addition of either effects, when cumarat-CoA-ligase and coniferaldehyde-5-hydroxylase are overexpressed or downregulated in one transgenic plant. As a result the total lignin content is reduced about 52%, the part of cellulose
Cumarat Aus Cinnamat
  1. 13.1 Proposed biosynthetic pathway of lignin synthesis in trees. PAL Phenylalanine-ammoniumlyase, C4H cinnamate 4-hydroxylyase, C3H 4-coumarate 3-hydroxylase, 4CL 4-cou-marate-CoA ligase, CCoAOMT caffeoyl CoA O-methyltransferase, CCR cinnamoyl-CoA reductase, CAld5H coniferaldehyde 5-hydroxylase, AldOMT 5-hydroxyconiferaldehyde O-methyltransferase, SAD sinapyl alcohol dehydrogenase, CAD cinnamyl alcohol dehydrogenase
  2. 13.1 Proposed biosynthetic pathway of lignin synthesis in trees. PAL Phenylalanine-ammoniumlyase, C4H cinnamate 4-hydroxylyase, C3H 4-coumarate 3-hydroxylase, 4CL 4-cou-marate-CoA ligase, CCoAOMT caffeoyl CoA O-methyltransferase, CCR cinnamoyl-CoA reductase, CAld5H coniferaldehyde 5-hydroxylase, AldOMT 5-hydroxyconiferaldehyde O-methyltransferase, SAD sinapyl alcohol dehydrogenase, CAD cinnamyl alcohol dehydrogenase increased up to 30% and the proportion of sinapyl to coniferyl units increased up to 64% (Li and Quiros 2003). 4. The proportion of sinapyl to coniferyl units increased, when the coniferylalde-hyd-5-hydroxylase gene is regulated by the promoter of the cinnamic acid hydrolase gene. The transgenic poplar trees show no phenotypical changes compared to control plants; furthermore their wood has a higher decomposition efficiency than normal poplar trees (Huntley et al. 2003).

The interest of industries in genetically modification of forest trees is extremely higher in the United States than in Europe, caused by geographical conditions and therefore expansion of the forest industry. Furthermore, commercialization of transgenic trees in Europe seems to have fewer chances, due to their risk assessment, e.g. durability and high spreading potential (Sauter and Hiisung 2005).

13.2.1.3 Glucoside

The ingredients for cosmetic creams, lotions, powder, perfumes, lipstick or makeup come from a variety of sources, for example antioxidants, alcohol, oil and also polymers. Polymers serve in hair-setting products, as binders in skin creams and to keep sunscreens from washing off. One example is a-D-glucosylglycerol (a-GG), which is used as an anti-aging agent and moisture-regulating compound (Da Costa et al. 1998). a-GG can be produced by chemical as well as by enzymatic methods and was naturally found in microorganisms as a compatible solute for providing some protection against stresses due to high salt concentrations, heat and UV radiation. It is also useful as an alternative sweetener in food stuffs, because of its low caloric value. The microbial synthesis of a-GG is presently not a mature process, because it does not allow the production of a-GG as a bulk chemical. The achievable concentrations are very low and also the productivity of three days is not advantageous for industrial production (Roder et al. 2005). However, a-GG is enzymatically synthesized (Godl et al. 2008) by using sucrose phosphorylase to convert sucrose with glucosyl and glycerol into a-GG, which is isolated by chro-matographic methods with a yield greater than 70%. Up to now, besides bacteria, GG accumulation has only been reported for the plants Lillium japonicum (Kaneda et al. 1984) and Myrothamnus flabellifolia (Bianchi et al. 1993); however nothing is known about the metabolic pathway of GG in these plants. In unpublished data GG accumulation was established in Arabidopsis by expression of the ggpPS (glucosylglycerol phosphate phosphatase/synthase) gene from the g-proteobacterium Azotobacter vinelandii. Transgenic plants accumulated GG up to 30 mmol/g fresh weight. However, beside increased salt tolerance, plants with higher GG content also showed growth retardation, which is not observed in plants with low GG accumulation (1-2 mmol/g fresh weight; Klahn et al. 2008). The growth retardations might be caused by the interference of GG synthesis with trehalose biosynthesis and in turn also other carbohydrates. The improvement of the GG synthesis in plants needs more investigation by regulated gene expression.

Was this article helpful?

0 0

Post a comment